Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of the positive silver image, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith, a soluble silver complex is obtained by reaction of a silver halide solvent with the unexposed and undeveloped silver halide of said emulsion. The photosensitive silver halide emulsion is developed with a processing composition which may be spread between the photosensitive element comprising the silver halide emulsion and a second element which may comprise a suitable silver-precipitating layer. The processing composition effects development of the latent image in the emulsion and, substantially contemporaneously therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate, with undeveloped silver halide. This soluble silver complex is, at least in part, transported in the direction of the print-receiving layer and the silver thereof is largely precipitated in the silver precipitating element to form a positive image thereon. Procedures of this description are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. See, also, Edwin H. Land, One Step Photography, Photographic Journal, Section A, pp. 7-15, January 1950.
Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color, such as red or green or blue, and by viewing the reversed or positive silver image formed by transfer to a transparent print-receiving element through the same or a similar screen which is suitably registered with the reversed positive image carried by the print-receiving layer.
As examples of suitable film structures for employment in additive color photography, mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,427; 3,615,428; 3,615,429; 3,615,426; and 3,894,871.
Many film assemblies disclosed in the above-indicated patents find particular utility in cine film systems such as, for example the system described in U.S. Pat. No. 3,615,127 which includes a compact film cassette or container adapted to allow exposure of a film assemblage retained therein, subsequent processing of the film unit to provide the desired image record and projection of the resultant image record. Thus, the film assemblage may be exposed, processed, dried if necessary, and projected without transferring the film from its original container. The cine film system of U.S. Pat. No. 3,615,127 includes a film processing station whereupon the exposed film strip is transported from a first storage reel, past an applicator where a moist processing composition adapted to develop to a visible condition images recorded on the film is applied and thence to a second storage reel.
Copending application Ser. No. 649,201, filed Jan. 14, 1976, abandoned and replaced by U.S. application Ser. No. 69,282 filed Aug. 24, 1979 (commonly assigned) discloses and claims a receiving element particularly suited for use in an additive color photographic diffusion transfer film unit which comprises a transparent support carrying an additive color screen and a layer comprising noble metal silver-precipitating nuclei and a polymer; wherein the nuclei are present in a level of about 0.1-0.3 mgs/ft.sup.2, and said polymer is present at a level of from about 0.5 to 5 times the coverage of said nuclei. Preferably, the noble metal is obtained by reduction of a noble metal salt or complex, and more preferably, the noble metal is palladium.
Copending applications Ser. No. 897,943 and 897,942, both filed Apr. 4, 1978, and commonly assigned disclose and claim image-receiving elements particularly suited for use in additive diffusion transfer film units of the type described which comprise noble metal silver precipitating nuclei and, respectively, gelatin and hydroxyethyl cellulose and gelatin and polyvinyl alcohol at specified levels and ratios of the polymers.
The additive diffusion transfer film units disclosed above are processed by an aqueous alkaline processing composition which includes a silver halide developing agent and a silver halide solvent.
Silver halide solvents useful in forming the desired soluble complex with unexposed silver are well known and, for example, may be selected from the alkali metal thiosulfates, particularly sodium or potassium thiosulfates, or the silver halide solvent may be a cyclic imide, such as uracil, in combination with a nitrogenous base as taught in U.S. Pat. No. 2,857,273 issued Oct. 21, 1975, to Edwin H. Land or a pseudo uracil, such as the 4,6-dihydroxyprimidines as taught in U.S. Pat. No. 4,126,459, issued Nov. 21, 1978.
The processing composition may contain a thickening agent, such as an alkali metal carboxymethyl cellulose or hydroxyethyl cellulose, in a quantity and viscosity grade adapted to facilitate application of the processing composition. The processing composition may be left on the processed film or removed, in accordance with known techniques, as is most appropriate for the particular film use. The requisite alkalinity, e.g., a pH of 12-14, is preferably imparted to the processing composition by an alkaline material such as sodium, potassium and/or lithium hydroxide.
Suitable silver halide developing agents may be selected from amongst those known in the art, and may be initially positioned in a layer of the photosensitive element and/or in the processing composition. Organic silver halide developing agents are generally used, e.g., organic compounds of the benzene or naphthalene series containing hydroxyl and/or amino groups in the para-or ortho-positions with respect to each other, such as hydroquinone, tert-butyl hydroquinone, toluhydroquinone, p-aminophenol, 2,6-dimethyl-4-aminophenol, 2,4,6-triaminophenol, etc. If the additive color transparency is one which is not washed after processing to remove unused silver halide developing agent, development reaction products, etc., the silver halide developing agent, development reaction products, etc., should not give rise to colored reaction products which might stain the image or which, either unreacted or reacted, might adversely affect the stability and sensitometric properties of the final image. Particularly useful silver halide developing agents having good stability in alkaline solution are substituted reductic acids, particularly tetramethyl reductic acid, as disclosed in U.S. Pat. No. 3,615,440 issued Oct. 26, 1971 to Stanley M. Bloom and Richard D. Cramer.
The above-mentioned additive diffusion transfer film units, particularly when employed as an elongated strip in a motion picture mode, often are processed in a manner which does not provide for washing or otherwise removing or neutralizing the processing composition in the layers of the film unit. In some instances, with time the projected image exhibited dark areas of various shapes and disposition which has been attributed to crystal formation or a salting out of one or more of the processing composition reagents or processing by products in the film unit. While not intending to be bound by theory, it is believed that the crystals are derived from developer and/or silver halide solvent. These crystals are generally found at or adjacent to the surface of the film unit. Copending Application Ser. No. 897,940, filed Apr. 4, 1978 (commonly assigned), now U.S. Pat. No. 4,168,166, is directed to the employment of a borate in the processing composition which substantially obviates the above-mentioned crystal problem.
Another defect has been observed which is also believed to involve crystal formation but this time the crystals are located within the film unit structure rather than being associated with the surface and appear as white spots in the projected image after the film has been processed and stored for some time. It is believed that the crystal growth destroys the integrity of the positive image layer; in effect making a "hole" in the positive image layer. While the invention of copending application Ser. No. 897,940 is effective with the surface associated defect, it does not appear to be effective against the interior crystal formation.